[ENH] Weighted regression

[QuantumPlumber]

This PR covers numeric dataset weighting for XGBoost and LightGBM, and associated tests. Users can now provide a json dictionary of histogram bin edges and associated weights, which will be used to map the labels to weights. The histogram bins are left edge -- overflows are assigned to the last bin's weight, and underflows to the first bin's weight.

[paxcema]

Mostly minor things to comment on. The one big change still needed is the merge with staging then ensuring CI installs lightgbm, but not making it a default dep just yet.

The rest I went through on my end, and it works just as advertised 🙌